Myrtle is trying to justify the fact that she married George Wilson in the first place.

Clothes are used as one of many symbols of the differences between the classes in The Great Gatsby. When Myrtle leaves her over-the-garage home with George, she is wearing "a brown flowered muslin, which stretched tight over her rather wide hips" - hardly an attractive appearance. Upon arriving at the apartment she shares with Tom, Myrtle changes into "an elaborate afternoon dress of cream-colored chiffon, which gave out a continual rustle as she swept about the room."

Myrtle wants the luxuries and refinements that Tom's money can buy. She married George because she met him first and was impressed with his "breeding." It appeared that he had an elegant suit to wear for their wedding. Myrtle was devastated to learn that it was borrowed from a friend - the first indication that she was making a mistake in marrying George.

In the city, at the apartment she shares with Tom when he wants to meet with her, Myrtle attempts to justify her marital infidelity (in addition to her decision to marry her husband, George, as the other commenter stated). Myrtle wants sympathy for having married George; when her friend, Mrs. McKee, talks about nearly marrying someone "'below [her],'" Myrtle retorts, "'Yes, but [...] at least you didn't marry him.'" She is bitter because she did marry a man who she believes to be well beneath her in every way. She declares that she was never crazy about George, as her sister, Catherine, insists, only that she was crazy when she married him to begin with. Myrtle says, "I knew right away I made a mistake" because George had borrowed his suit for the wedding; he'd seemed like someone with "breeding" to her, but she soon learned that "'he wasn't fit to lick [her] shoe.'" Catherine claims, "'She really ought to get away from him.'" Catherine says that they've been living over George's garage for eleven years, as though this were justification enough for Myrtle to cheat on her husband. Myrtle's arguments seem designed to justify her disloyalty to George.